Three-phase inductive motors, as well as many other three-phase loads, are often inoperable or susceptible to damage due to power supply faults such as power loss, phase loss, and phase reversal. More specifically, a momentary power loss allows a motor to decelerate which increases the degree of slip between the rotor and the alternating current passing through the stator. Upon restoration of power, the rotor tries to quickly recover its original slip relationship with the current. In the rotor's attempt to do so, high torques are developed which can damage the rotor shaft or other components associated with it.
In a phase loss, current is supplied to the motor through only two of the three supply lines. When this happens, the motor tries to compensate for the inactive phase by conducting additional current through the stator windings which are connected to the remaining two active phases. As a result of the additional current, the active windings can overheat to a destructively high temperature.
In reaction to a phase reversal, often referred to as reverse phase rotation, the motor rotates in a direction opposite its normal rotation. This is often caused by improperly matching the motor leads to the power supply. Depending on the specific application, reverse rotation can make a critical motor driven oil pump inoperative or unscrew an impeller from a threaded drive shaft, either of which can cause extensive damage.
Although a wide variety of fault detectors for use in three-phase circuits are presently available, many do not distinguish a phase reversal from other faults such as a phase or power loss. Moreover, in many applications, their timely response to faults is inadequate. Some detectors respond too slowly to critical faults such as a momentary power loss, where damage can quickly occur if the detector doesn't interrupt the power supply before power is restored. Other detectors respond too quickly to less critical faults where a slower response is desirable to reduce the effects of false readings due to electrical noise.
Still other detectors often include many electrical components which not only increase the cost of the detector but are often unnecessary. This is especially true when the detector is associated with a system that already includes a microcomputer based controller. In such systems, the microcomputer could often be used to further serve the function of much of the detector circuitry.
Therefore, it is an object of this invention to provide a phase reversal detector that includes a microcomputer instead of numerous discrete electrical components.
Another object is to provide a three-phase fault detector that detects and distinguishes among several faults including phase reversal, phase loss, and power loss.
Yet another object is to provide a fault detector whose response time to various faults is accurately and independently adjustable.
Still another object is to provide a three-phase fault detector having a sampling time that is accurately synchronized with the three-phase circuit by using an interrupt signal indirectly generated by the current in one of the three phases.
These and other objects of the invention will be apparent from the attached drawings and the description of the preferred embodiment which follow hereinbelow.